When a signal whose band has a short wavelength, such as millimeter waves or microwaves, which is typically used for car radar and high-seed wireless communication system, is transmitted from and received at an antenna by using a transmitter-receiver circuit, a waveguide may be connected between the transmitter-receiver circuit and the antenna.
The transmitter-receiver circuit is integrated, for example, as a monolithic microwave integrated circuit (MMIC), and a planar transmission line such as a microstrip line and a coplanar line is used for a transmission line on the transmitter-receiver circuit side. The transmission mode of a signal is different between such a transmission line on a transmitter-receiver circuit side and a waveguide. Thus, when a waveguide is connected between a transmitter-receiver circuit and an antenna, a waveguide converter is used to convert the transmission mode so as to be suitable for the transmission line on a transmitter-receiver circuit side and the waveguide, respectively.
In regard to waveguide converters, the following related art is known. That is, a microstrip line—waveguide converter is comprised of a waveguide, a first conductor layer, a dielectric substrate, and a ground conductor layer. The first conductor layer is comprised of a microstrip line that has a patch pattern formed on an end, a ground conductor pattern that surrounds the patch pattern, and via holes that connect the ground conductor pattern and the ground conductor layer. Then, the waveguide, the first conductor layer, the dielectric substrate, and the ground conductor layer are stacked from the top in the listed order at a position where the center of the opening of a waveguide and the center of the patch pattern overlap with each other. A number of via holes are formed so as to surround the periphery of the opening of the waveguide.
Moreover, the following related art is also known. That is, a waveguide/strip line converter is provided with: a dielectric substrate having a first surface that closes the rectangular opening of a waveguide; a shorting plate formed on a second surface of a dielectric substrate to short the waveguide; a matching element formed on a first surface of the dielectric substrate; and a strip line that is formed in an incision of the shorting plate and is electromagnetically coupled to the matching element. The matching element is shaped so as to surround a non-formation area, and has an asymmetrical shape with reference to a direction parallel to the long sides of the opening.
Furthermore, the following related art is also known. That is, a waveguide/strip line converter is comprised of a rectangular waveguide and a dielectric substrate. An aperture for guiding an electromagnetic wave is arranged on one end of the rectangular waveguide, and an end surface is arranged on the other end. The dielectric substrate is inserted into the rectangular waveguide from the side of the dielectric substrate in such a manner that the dielectric substrate exists in a direction orthogonal to the end surface of the rectangular waveguide and the mounted position viewed from the opening is at approximately the center of the aperture. Moreover, an approximately cross-shaped conductor pattern is arranged on the dielectric substrate, and one side of the conductor pattern is extended as a pattern to draw out a signal to the outside of the rectangular waveguide. The pattern to draw out a signal is formed as a strip line outside the rectangular waveguide. The electric field of an electromagnetic wave that is guided into the rectangular waveguide is coupled to the conductor pattern, and is converted to an electric signal by the conductor pattern and transmitted to the strip line.
The waveguide converter includes a conductor patch. The conductor patch has the function of emitting a signal that is transmitted through the transmission line on a transmitter-receiver circuit side to the waveguide, and has the function of emitting a signal that is transmitted through the waveguide to the transmission line on the transmitter-receiver circuit side.
It is necessary for the size of the conductor patch to be smaller than the opening of a waveguide that is determined according to an active frequency band. In order for the waveguide converter to achieve a good signal conversion performance in a desired frequency band, it is necessary to determine the shape and size of the conductor patch according to the wavelength of a signal determined by the dielectric constant or the like of the dielectric substrate that composes the transmission line on the transmitter-receiver circuit side.
When a rectangular-shaped conductor patch is provided for a waveguide converter, the waveguide converter may perform signal conversion in a desired active frequency if the length of sides of the conductor patch that is parallel with the transmission direction of a signal of the transmission line on the transmitter-receiver circuit side are set to be half the wavelength of the signal. However, half the wavelength of a signal that is transmitted through the dielectric substrate may be greater than the short sides of the opening of the waveguide when, for example, a low-level side of a recommended frequency band of the waveguide is used or when, for example, a dielectric substrate of a low dielectric constant is used. In order to achieve a good signal conversion performance in such a case by using a rectangular-shaped conductor patch, it is necessary for the shape of a conductor patch to be rectangular and longer in a direction of the long sides of the opening of the waveguide. However, depending on the length of the long sides of a conductor patch, a resonance that degrades the pass characteristic of a signal between the waveguide and the transmission line is caused near the active frequency band. For this reason, it is necessary to design the waveguide converter such that a resonance frequency that degrades the pass characteristic of the waveguide converter will not be caused near the active frequency band.
Moreover, when a resin whose pattern precision is poor is used, for example for the purpose of cost reduction, as a substrate material instead of ceramics, a pattern misalignment may be caused when a waveguide converter is manufactured.
FIG. 1 depicts the deterioration of a pass characteristic caused due to a pattern misalignment.
In FIG. 1, pass characteristics T1 and T2 of a waveguide converter are depicted with a scattering parameter S21 where a port 1 is on a waveguide side and a port 2 is on a transmission line side to which a transmitter-receiver circuit is connected.
As illustrated in FIG. 1, the pass characteristic T2 where pattern misalignment was caused when the waveguide converter was manufactured deteriorates at the center frequency of an active frequency band fc in comparison with the pass characteristic T1 where no pattern misalignment was caused. A resonance frequency fr2 that degrades the pass characteristic T2 is closer to the center frequency of an active frequency band fc in comparison with a resonance frequency fr1 that degrades the pass characteristic T1.
As described above, when a pass characteristic deteriorates at the center frequency of an active frequency band due to the pattern misalignment that was caused when the waveguide was manufactured, or when a resonance frequency that degrades the pass characteristic is misaligned and gets close to an active frequency band, a signal conversion performance of the waveguide converter deteriorates. Thus, it is necessary to design a waveguide converter in such a manner that the deterioration of a pass characteristic will be minimized and a required signal conversion performance will be secured even if the pattern precision of the waveguide converter at the time of manufacture is poor.